This application relates to semiconductor structures and processes, and particularly relates to group III-nitride quaternary and pentenary materials systems and methods such as might be used in blue or ultraviolet laser diodes and other similar semiconductors.
The development of the blue laser light source has heralded the next generation of high density optical devices, including disc memories, DVDs, and so on. FIG. 1 shows a cross sectional illustration of a prior art semiconductor laser devices. (S. Nakamura, MRS BULLETIN, Vol. 23, No. 5, pp. 37-43, 1998.) On a sapphire substrate 5, a gallium nitride (GaN) buffer layer 10 is formed, followed by an n-type GaN layer 15, and a 0.1 xcexcm thick silicon dioxide (SiO2) layer 20 which is patterned to form 4 xcexcm wide stripe windows 25 with a periodicity of 12 xcexcm in the GaN less than 1-100 greater than  direction. Thereafter, an n-type GaN layer 30, an n-type indium gallium nitride (In 0.1Ga0.9N) layer 35, an n-type aluminum gallium nitride (Al0.14Ga0.86N)/GaN MD-SLS (Modulation Doped Strained-Layer Superlattices) cladding layer 40, and an n-type GaN cladding layer 45 are formed. Next, an In0.02Ga0.98N/In0.15Ga0.85N MQW (Multiple Quantum Well) active layer 50 is formed followed by a p-type Al0.2Ga0.8N cladding layer 55, a p-type GaN cladding layer 60, a p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65, and a p-type GaN cladding layer 70. A ridge stripe structure is formed in the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65 to confine the optical field which propagates in the ridge waveguide structure in the lateral direction. Electrodes are formed on the p-type GaN cladding layer 70 and n-type GaN cladding layer 30 to provide current injection.
In the structure shown in FIG. 1, the n-type GaN cladding layer 45 and the p-type GaN 60 cladding layer are light-guiding layers. The n-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 40 and the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65 act as cladding layers for confinement of the carriers and the light emitted from the active region of the InGaN MQW layer 50. The n-type In0.1Ga0.9N layer 35 serves as a buffer layer for the thick AlGaN film growth to prevent cracking.
By using the structure shown in FIG. 1, carriers are injected into the InGaN MQW active layer 50 through the electrodes, leading to emission of light in the wavelength region of 400 nm. The optical field is confined in the active layer in the lateral direction due to the ridge waveguide structure formed in the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65 because the effective refractive index under the ridge stripe region is larger than that outside the ridge stripe region. On the other hand, the optical field is confined in the active layer in the transverse direction by the n-type GaN cladding layer 45, the n-type Al0.14Ga0.86N/GaN MD-SLS cladding layers 40, the p-type GaN cladding layer 60, and the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65 because the refractive index of the of the active layer is larger than that of the n-type GaN cladding layer 45 and the p-type GaN cladding layer 60, the n-type Al0.14Ga0.86N/GaN MD-SLS layer 40, and the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65. Therefore, fundamental transverse mode operation is obtained.
However, for the structure shown in FIG. 1, it is difficult to reduce the defect density to the order of less than 108 cmxe2x88x922, because the lattice constants of AlGaN, InGaN, and GaN differ sufficiently different from each other that defects are generated in the structure as a way to release the strain energy whenever the total thickness of the n-type In0.1Ga0.9N layer 35, the In0.02Ga0.98N/In0.15Ga0.85N MQW active layer 50, the n-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 40, the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65, and the p-type Al0.2Ga0.8N cladding layer 55 exceeds the critical thickness. The defects result from phase separation and act as absorption centers for the lasing light, causing decreased light emission efficiency and increased threshold current. The result is that the operating current becomes large, which in turn causes reliability to suffer.
Moreover, the ternary alloy system of InGaN is used as an active layer in the structure shown in FIG. 1. In this case, the band gap energy changes from 1.9 eV for InN to 3.5 eV for GaN. Therefore, ultraviolet light which has an energy level higher than 3.5 eV cannot be obtained by using an InGaN active layer. This presents difficulties, since ultraviolet light is attractive as a light source for the optical pick up device in, for example, higher density optical disc memory systems and other devices.
To better understand the defects which result from phase separation in conventional ternary materials systems, the mismatch of lattice constants between InN, GaN, and AlN must be understood. The lattice mismatch between InN and GaN, between InN and AlN, and between GaN and AlN, are 11.3%, 13.9%, and 2.3%, respectively. Therefore, an internal strain energy accumulates in an InGaAlN layer, even if the equivalent lattice constant is the same as that of the substrate due to the fact that equivalent bond lengths are different from each other between InN, GaN, and AlN. In order to reduce the internal strain energy, there is a compositional range which phase separates in the InGaAlN lattice mismatched material system, where In atoms, Ga atoms, and Al atoms are inhomogeneously distributed in the layer. The result of phase separation is that In atoms, Ga atoms, and Al atoms in the InGaAlN layers are not distributed uniformly according to the atomic mole fraction in each constituent layer. In turn, this means the band gap energy distribution of any layer which includes phase separation also becomes inhomogeneous. The band gap region of the phase separated portion acts disproportionately as an optical absorption center or causes optical scattering for the waveguided light. As noted above, a typical prior art solution to these problems has been to increase drive current, thus reducing the life of the semiconductor device.
As a result, there has been a long felt need for a semiconductor structure which minimizes lattice defects due to phase separation and can be used, for example, as a laser diode which emits blue or UV light at high efficiency, and for other semiconductor structures such as transistors.
The present invention substantially overcomes the limitations of the prior art by providing semiconductor structures and methods which substantially reduce defect densities by materially reducing phase separation between the layers of the structures. This in turn permits substantially improved emission efficiency. In general, the present invention utilizes group III-nitride quaternary and pentenary material systems and methods which may generally be described by the formula In1xe2x88x92xxe2x88x92yGaxAlyN1xe2x88x92zAsz, where x, y and z are not all zero. It will be appreciated that, where one of x, y or z is zero, a quaternary system results. However, if none of x, y or z are zero, and the sum of x and y does not total one, a pentenary material system results.
To reduce phase separation, it has been found possible to provide a semiconductor device with GaAlNAs layers having homogeneous Al content distribution as well as homogeneous As content distribution in each layer. In a light emitting device, this permits optical absorption loss and waveguide scattering loss to be reduced, resulting in a high efficiency light emitting device. By carefully selecting the amounts of Al and As, devices with at least two general ranges of band gaps may be produced, allowing development of light emitting devices in both the infrared and the blue/uv ranges.
In a first exemplary embodiment of a GaAlNAs quaternary material system in accordance with the present invention, sufficient homogeneity to avoid phase separation has been found when the Al content, represented by x, and the As content, represented by y, ideally satisfy the condition that 3.18(1xe2x88x92x)(1xe2x88x92y)+3.99(1xe2x88x92x)y+3.11x(1xe2x88x92y)+4xy nearly equals to a constant value. In a typical embodiment of a light emitting device, the constant value may be 3.18. The lack of phase separation results because the lattice constants of the constituent layers in the structure are sufficiently close to each, in most cases being nearly equal, that the generation of defects is suppressed.
A device according to a first embodiment of the present invention typically includes a first layer of GaAlNAs material of a first conductivity, a GaAlNAs active layer, and a layer of GaAlNAs material of an opposite conductivity successively formed on one another. By maintaining the mole fractions essentially in accordance with the formula 3.18(1xe2x88x92x)(1xe2x88x92y)+3.99(1xe2x88x92x)y+3.11x(1xe2x88x92y)+4xy nearly equals to a constant value, for example on the order of or nearly equal to 3.18, the lattice constants of the constituent layers remain substantially equal to each other, leading to decreased generation of defects.
In an alternative embodiment, the semiconductor structure is fabricated essentially as above, using a quaternary materials system to eliminate phase separation and promote homogeneity across the layer boundaries. Thus, as before, the first cladding layer is a first conduction type and composition of GaAlNAs, the active layer is GaAlNAs of a second composition, and the second cladding layer is an opposite conduction type of GaAlNAs having the composition of the first layer. However, in addition, the second cladding layer has a ridge structure. As before, the optical absorption loss and waveguide scattering loss is reduced, leading to higher efficiencies, with added benefit that the optical field is able to be confined in the lateral direction in the active layer under the ridge structure. This structure also permits fundamental transverse mode operation.
In a third embodiment of the invention, suited particularly to implementation as a laser diode, the semiconductor structure comprises a first cladding of a first conduction type of an Ga1xe2x88x92x1Alx1N1xe2x88x92y1Asy1 material, an active layer of an Ga1xe2x88x92x2Alx2N1xe2x88x92y2Asy2 material, and a second cladding layer of an opposite conduction type of an Ga1xe2x88x92x3Ax3N1xe2x88x92y3Asy3 material, each successively formed on the prior layer. In such a materials system, x1, x2, and x3 define the Al content and y1, y2, and y3 define the As content. Moreover, x1, y1, x2, y2, x3, and y3 have a relationship of 0 less than x1 less than 1, 0 less than x2 less than 1, 0 less than x3 less than 1, 0 less than y1 less than 1, 0 less than y2 less than 1, 0 less than y3 less than 1, 0.26x1+37y1 less than =1, 0.26x2+37y2 less than =1, 0.26x3+37y3 less than =1, EgGaN(1xe2x88x92x1)(1xe2x88x92y1)+EgGaAs(1xe2x88x92x1)y1+EgAlNx1(1xe2x88x92y1)+EgAlAsx1y1 greater than EgGaN(1xe2x88x92x2)(1xe2x88x92y2)+EgGaAs(1xe2x88x92x2)y2+EgAlNx2(1xe2x88x92y2)+EgAlAsx2y2, and EgGaN(1xe2x88x92x3)(1xe2x88x92y3)+EgGaAs(1xe2x88x92x3)y3+EgAlNx3(1xe2x88x92y3)+EgAlAsx3y3 greater than EgGaN(1xe2x88x92x2)(1xe2x88x92y2)+EgGaAs(1xe2x88x92x2)y2+EgAlNx2 (1xe2x88x92y2)+EgAlAsx2y2, where EgGaN, EgGaAs, EgAlN, and EgAlAS are the band gap energy of GaN, GaAs, AlN, and AlAs, respectively.
To provide a reproducible semiconductor structure according to the above materials system, an exemplary embodiment of GaAlNAs layers have Al content, x, and As content, y, which satisfy the relationship 0 less than x less than 1, 0 less than y less than 1 and 0.26x+37y less than =1. As before, this materials system permits reduction of the optical absorption loss and the waveguide scattering loss, resulting in a high efficiency light emitting device. Moreover, the band gap energy of the GaAlNAs of an active layer becomes smaller than that of the first cladding layer and the second cladding layer when x1, y1, x2, y2, x3, and y3 have a relationship of 0 less than x1 less than 1, 0 less than x2 less than 1, 0 less than x3 less than 1, 0 less than y1 less than 1, 0 less than y2 less than 1, 0 less than y3 less than 1, 0.26x1+37y1 less than =1, 0.26x2+37y2 less than =1, 0.26x3+37y3 less than =1, EgGaN(1xe2x88x92x1)(1xe2x88x92y1)+EgGaAs(1xe2x88x92x1)y1+EgAlNx1(1xe2x88x92y1)+EgAlAsx1y1 greater than EgGaN(1xe2x88x92x2)(1xe2x88x92y2)+EgGaAs(1xe2x88x92x2)y2+EgAlNx2(1xe2x88x92y2)+EgAlAsx2y2, and EgGaN(1xe2x88x92x3)(1xe2x88x92y3)+EgGaAs(1x3)y3+EgAlNx3(1xe2x88x92y3)+EgAlAsx3y3 greater than EgGaN(1xe2x88x92x2)(1xe2x88x92y2)+EgGaAs(1xe2x88x92x2)y2+EgAlNx2(1xe2x88x92y2)+EgAlAsx2y2. Under these conditions, the injected carriers are confined in the active layer. In at least some embodiments, it is preferable that the third light emitting device has a GaAlNAs single or multiple quantum well active layer whose Al content, xw, and As content, yw, of all the constituent layers satisfy the relationship of 0 less than xw less than 1, 0 less than yw less than 1, and 0.26xw+37yw less than =1.
One of the benefits of the foregoing structure is to reduce the threshold current density of a laser diode. This can be achieved by use of a single or multiple quantum well structure, which reduces the density of the states of the active layer. This causes the carrier density necessary for population inversion to become smaller, leading to a reduced or low threshold current density laser diode.
It is preferred that in the third light emitting device, the condition of 3.18(1xe2x88x92xs)(1xe2x88x92ys)+3.99(1xe2x88x92xs)ys+3.11xs(1xe2x88x92ys)+4xsys nearly equals to a constant valuexe2x80x94on the order of or near 3.18xe2x80x94is satisfied, wherein xs and ys are the Al content and the As content, respectively, in each the constituent layers. As before, this causes the lattice constants of the each constituent layers to be nearly equal to each other, which in turn substantially minimizes defects due to phase separation.
In a fourth embodiment of the present invention, the semiconductor structure may comprise a first cladding layer of a first conduction type of a material Ga1xe2x88x92x1Alx1N1xe2x88x92y1Asy1, a Ga1xe2x88x92x2Alx2N1xe2x88x92y2Asy2 active layer, and a second cladding layer of an opposite conduction type of a material Ga1xe2x88x92x3Alx3N1xe2x88x92y3Asy3, each successively formed one upon the prior layer. In addition, the second cladding layer has a ridge structure. For the foregoing materials system, x1, x2, and x3 define the Al content, y1, y2, and y3 define the As content, and x1, y1, x2, y2, x3, and y3 have a relationship of 0 less than x1 less than 1, 0 less than x2 less than 1, 0 less than x3 less than 1, 0 less than y1 less than 1, 0 less than y2 less than 1,0 less than y3 less than 1, 0.26x1+37y1 less than =1, 0.26x2+37y2 less than =1, 0.26x3+37y3 less than =1, EgGaN(1xe2x88x92x1)(1xe2x88x92y1)+EgGaAs(1xe2x88x92x1)y1+EgAlNx1(1xe2x88x92y1)+EgAlAsx1y1 greater than EgGaN(1xe2x88x92x2)(1xe2x88x92y2)+EgGaAs(1xe2x88x92x2)y2+EgAlNx2(1xe2x88x92y2)+EgAlAsx2y2, and EgGaN(1xe2x88x92x3)(1xe2x88x92y3)+EgGaAs(1xe2x88x92x3)y3+EgAlNx3(1xe2x88x92y3)+EgAlAsx3y3 greater than EgGaN(1xe2x88x92x2)(1xe2x88x92y2)+EgGaAs(1xe2x88x92x2)y2+EgAlNx2(1xe2x88x92y2)+EgAlAsx2y2, where EgGaN, EgGaAsEgAlN, and EgAlAs are the band gap energy of GaN, GaAs, AlN, and AlAs, respectively.
As with the prior embodiments, each of the GaAlNAs layers have a homogeneous Al and As content distribution, which can be obtained reproducibly when Al content, x, and As content, y, of each GaAlNAs layer satisfies the relationship 0 less than x less than 1, 0 less than y less than 1 and 0.26x+37y less than =1. The band gap energy of the GaAlNAs active layer becomes smaller than that of the first cladding layer and the second cladding layer when x1, y1, x2, y2, x3, and y3 have a relationship of x1, y1, x2, y2, x3, and y3 have a relationship of 0 less than x1 less than 1, 0 less than x2 less than 1, 0 less than x3 less than 1, 0 less than y1 less than 1, 0 less than y2 less than 1, 0 less than y3 less than 1, 0.26x1+37y1 less than =1, 0.26x2+37y2 less than =1, 0.26x3+37y3 less than =1, EgGaN(1xe2x88x92x1)(1xe2x88x92y1)+EgGaAs(1xe2x88x92x1)y1+EgAlNx1(1xe2x88x92y1)+EgAlAsx1y1 greater than EgGaN(1xe2x88x92x2)(1xe2x88x92y2)+EgGaAs(1xe2x88x92x2)y2+EgAlNx2(1xe2x88x92y2)+EgAlAsx2y2, and EgGan(1xe2x88x92x3)(1xe2x88x92y3+EgGaAs(1xe2x88x92x3)y3+EgAlNx3(1xe2x88x92y3)+EgAlAsx3y3 greater than EgGaN(1xe2x88x92x2)(1xe2x88x92y2)+EgGaAs(1xe2x88x92x2)y2+EgAlNx2(1xe2x88x92y2)+EgAlAsx2y2. Similar to the prior embodiments, the injected carriers are confined in the active layer and the optical field is confined in the lateral direction in the active layer under the ridge structure, producing a fundamental transverse mode operation.
Also similar to the prior embodiments, the fourth embodiment typically includes a GaAlNAs single or multiple quantum well active layer whose Al content, xw, and As content, yw, of all the constituent layers satisfy the relationship of 0 less than xw less than 1, 0 less than yw less than 1, and 0.26xw+37yw less than =1. Also, the condition 3.18(1xe2x88x92xs)(1xe2x88x92ys)+3.99(1xe2x88x92xs)ys+3.11xs(1xe2x88x92ys)+4xsys nearly equals to a constant value, for example on the order of or near 3.18 is typically satisfied, where xs and ys are the Al content and the As content, respectively, in each constituent layer. Similar parameters apply for other substrates, such as sapphire, silicon carbide, and so on.
In the Group-III nitride materials, InN is also attractive for the application to visible light emitting devices, visible light detectors, and high power transistor devices because of its relativlely wide band gap and direct band gap. The same structural design concepts for GaAlNAs material are also can be applied to the semocinductor devices using the other material systems such as InGaNAs, AlInNAs.
In the case of the semiconductor device using InGaNAs material system, each of the InGaNAs layers have a homogeneous In content, Ga content, N content and As content distribution, which can be obtained reproducibly when Ga content, x, As content, y, of each InGaNAs layer satisfies the relationship 0 less than =x less than =1, 0 less than =y  less than =1, and x/0.2+y/0.9 less than =1 or 1.25xxe2x88x928.33y greater than =1.
In the case of the semiconductor device using AlInNAs material system, each of the AlInNAs layers have a homogeneous Al content, In content, N content, and As content distribution, which can be obtained reproducibly when In content, x, As content, y, of each AlInNAs layer satisfies the relationship 0 less than =x less than =1, 0 less than =y less than =1, and 1 less than =x/0.1+y/0.02 or x/0.9xe2x88x922.22y greater than =1.
The foregoing results may be achieved with conventional processing temperatures and times, typically in the range of 500xc2x0 C. to 1000xc2x0 C. See xe2x80x9cGrowth of high optical and electrical quality GaN layers using low-pressure metalorganic chemical vapor depositionxe2x80x9d, Appl Phys. Lett. 58(5), Feb. 4, 1991 p. 526 et seq.
The present invention may be better appreciated by the following Detailed Description of the Invention, taken together with the attached Figures.